Somatic sensations such as warming, cooling, gentle touch and pain are each initiated by activation of sensory neurons. Specific types of sensory neurons, whose cell bodies are located in dorsal root and trigeminal ganglia, subserve different sensory modalities. Specialized sensory neurons called nociceptors are responsible for the transduction of painful thermal and mechanical stimulation of the skin. Knowledge about molecules and ion channels that are necessary for the normal transduction of painful thermal and mechanical stimuli is still incomplete. It has been postulated that thermosensitive ion channels of the TRP family are important for the transduction of noxious heat or cold by nociceptive sensory neurons (Jordt et al., 2003). The most complete evidence exists for the capsaicin activated ion channel TRPV1 that can be activated by thermal stimuli in the noxious range. Mice lacking TRPV1 have altered pain behavior and do not respond to the noxious irritant capsaicin. An important feature of pain is the fact that injury and inflammation leads to heightened sensitivity to stimuli that would normally be only mildly painful. This phenomenon is called hyperalgesia, and the prevention of hyperalgesia is a hallmark of effective analgesia. TRPV1 may become an important analgesic target because this channel is required for the expression of thermal hyperalgesia provoked by inflammation (Caterina et al., 2000; Davis et al., 2000).
Moreover, molecules up-regulated in inflamed tissue such as nerve growth factor (NGF) can sensitize peripheral nociceptors to thermal stimuli. NGF signaling via its receptor tyrosine kinase TrkA constitutes a physiological mediator of inflammatory hyperalgesia. It has been known for many years that the dorsal root ganglion (DRG) neurons that require NGF are all nociceptors. NGF can produce a profound and long lasting thermal and mechanical hyperalgesia in man and animals. NGF can also potentiate TRPV1 mediated and noxious heat activated ionic currents in isolated DRG neurons. Indeed, NGF injected into animals produces thermal hyperalgesia that requires the presence of TRPV1 (Chuang et al., 2001).
Around half of the nociceptors in the adult DRG possess TrkA receptors; the remainder, defined by the expression of c-Ret, downregulate TrkA during early postnatal development. The receptor tyrosine kinase c-Ret mediates signals elicited by the glial-derived neurotophic factor (GDNF) ligand family. The c-Ret receptor and its co-receptors GFRα2 and 3 are present in nociceptive neurons, some of which are heat sensitive and express TRPV1 receptors. Indeed, there is some evidence for a role of the GDNF family ligands neurturin and artemin in regulating noxious heat transduction by sensory neurons (Malin et al., 2006).
In addition to the Trk and c-Ret receptors, sensory neurons are known to express other receptor tyrosine kinases like c-Kit, the receptor for stem cell factor (SCF). Thus, the European patent application No EP 2 068 152 discloses that the central role for SCF and its receptor, c-Kit, in tuning the responsiveness of sensory neurons to natural stimuli and that c-Kit can now be grouped with a small family of receptor tyrosine kinases, including c-Ret and TrkA, that control the transduction properties of sensory neurons. Said patent application claims the use of a c-kit receptor antagonist such as the small molecule drug imatinib for treating or preventing a disorder selected from pain, hyperalgesia and inflammatory pain.
However, no investigation on FLT3 receptor pathway (another tyrosine kinase receptor) has been made until now on pain regulation.